/*
 * (C) APUE
 * Initialize a daemon process
 */
#include "apue.h"
#include <syslog.h>
#include <fcntl.h>
#include <sys/resource.h>

void daemonize(const char *cmd)
{
	int i, fd0, fd1, fd2;
	pid_t pid;
	struct rlimit rl;
	struct sigaction sa;

	/*
	 * Clear file creation mask
	 */
	umask(0);

	/*
	 * Get maximum number of file descriptors.
	 */
	if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
		err_quit("%s: can't get file limit", cmd);

	/*
	 * Fork a new process
	 */
	if ((pid = fork()) < 0)
		err_quit("%s: can't fork", cmd);
	else if (pid > 0)	/* parent */
		exit(0);

	/* now in child */
	/* create a new session. */
	setsid();

	/*
	 * Ensure future opens won't allocate controlling TTYs
	 */
	sa.sa_handler = SIG_IGN;
	sigemptyset(&sa.sa_mask);
	sa.sa_flags = 0;
	if (sigaction(SIGHUP, &sa, NULL) < 0)	/* SIGHUP usually in daemon process for reloading configure files, here just ignore it */
		err_quit("%s: can't ignore SIGHUP", cmd);
	/* fork again to make the daemon process being not the session leader, avoid to get the TTYs */
	if ((pid = fork()) < 0)
		err_quit("%s: can't fork", cmd);
	else if (pid > 0)	/* parent */
		exit(0);

	/*
	 * Change the current working directory to the root so
	 * we won't prevent file systems from being unmounted.
	 */
	if (chdir("/") < 0)
		err_quit("%s: can't change directory to /", cmd);

	/* 
	 * Close all open file descriptors. 
	 */
	if (rl.rlim_max == RLIM_INFINITY)
		rl.rlim_max = 1024;
	for (i = 0; i < rl.rlim_max; i++)
		close(i);

	/*
	 * Attach file descriptors 0, 1, 2 to /dev/null
	 */
	fd0 = open("/dev/null", O_RDWR);
	fd1 = dup(0);
	fd2 = dup(0);

	/*
	 * Initialize the log file
	 */
	openlog(cmd, LOG_CONS, LOG_DAEMON);
	if (fd0 != 0 || fd1 != 1 || fd2 != 2)
	{
		syslog(LOG_ERR, "unexpected file descriptors %d %d %d", fd0, fd1, fd2);
		exit(1);
	}
}
